1. Field of the Invention
The present invention relates to a support structure in a crank mechanism and a component constituting the crank mechanism which converts reciprocating motion of a piston of an engine or the like to rotary motion by means of a crank pin, a crank arm and a crank shaft via a connecting bar.
2. Description of the Background Art
A crank mechanism converting the reciprocating motion of a piston to the rotary motion by means of a crank pin, a crank arm and a crank shaft via a connecting bar has conventionally been used in various fields. Typical application thereof is found, e.g., in two-wheeled vehicles, special vehicles, general-purpose two-cycle engine vehicles, and four-cycle engine vehicles. A support structure in the crank mechanism for these automobile engines includes a large-end bearing (provided between a connecting bar and a crank shaft and serving as a stress transmitting mechanism), a small-end bearing (provided between the connecting bar and a piston and serving as a stress transmitting mechanism), a crank shaft main bearing, and others. The support structure in the crank mechanism refers to a structure including these large-end and small-end bearings, crank shaft main bearing and others, and enabling conversion of the reciprocating motion of the piston to the rotary motion of the crank shaft.
For the support structure in the crank mechanism, a needle roller and cage assembly has conventionally been used to reduce size, weight and friction of the structure (see, e.g., Japanese Patent Laying-Open Nos. 11-101247 and 8-4774).
For the needle rollers in such a bearing of the support structure in the crank mechanism, a nitrided article of SUJ2 defined by JIS (Japanese Industrial Standard) has been used, and the needle roller and cage assembly having a large roller diameter has been used to ensure resistance to impact load.
The main bearing arranged at each end of the crank shaft rotatably supports the crank shaft that is driven to rotate by the large-end bearing. A deep groove ball bearing, for example, is used for the main bearing. This main bearing supporting the crank shaft experiences heavy impact load, as with the large-end and small-end bearings.
There is conventionally known a thermal treatment method ensuring a long life against rolling fatigue of a common bearing member, wherein a surface layer of the bearing member is subjected to a carbonitriding process, with an ammonia gas additionally mixed into an ambient RX gas at the time of heating. The carbonitriding process can harden the surface layer, generate residual austenite in the microstructure, and elongate a rolling fatigue life to some extent.
The support structure in the crank mechanism is required to have mechanical properties (1) ensuring a long life against rolling fatigue, (2) improving anti-crack strength, and (3) suppressing an increase in rate of secular dimensional change. For the future support structure in the crank mechanism to be subjected to heavier load and higher speed, mechanical properties ensuring its use under the conditions of heavier load and higher temperature than before is all the more required.